The public have been warned to be on their guard against the threat from anthrax following a series of incidents in the United States.

The Home Office said there was no intelligence to suggest there was a "specific threat" to Britain from biological or chemical attack.

But a Scotland Yard spokeswoman urged people to be "vigilant".

On Tuesday morning a sorting office in Liverpool was evacuated after white powder fell out of a package. The substance is being tested.

Only those involved in a specific incident would be tested

City of London Police are also checking a suspicious package at the Stock Exchange.

A Metropolitan Police spokeswoman said: "If people are suspicious of anything they have received in the mail or by hand they should contact police.

"The police will advise and them. We have asked people to be vigilant."

Meanwhile the government is stockpiling antibiotics to treat anthrax as it prepares to reveal its contingency plans for tackling bioterrorism.

Later this week guidelines drawn up by the Department of Health and the Home Office are to be sent to the NHS and local authorities advising them what to do in the event of an anthrax attack .

If people are suspicious of anything they have received in the mail or by hand they should contact police.

Scotland Yard spokeswoman

A baby boy and a 73-year-old man are the latest to be infected with anthrax in the US.

Packages have been sent to two media groups, one in Florida and one in New York, and to a leading senator in Washington DC.

Three people have been tested in Britain, after working in the US buildings where spores were detected. They are still awaiting results.

'Don't panic'

UK Government officials are keen to avoid panic.

Postal workers are in the frontline and Dave Joyce, chairman of the health safety committee of the Communication Workers' Union, said officials were trying to avoid panic within the Royal Mail.

He told BBC Radio 4's Today programme: "Members are being briefed and given adequate information and instruction as to the risk posed by anthrax, and what they should do if a suspect package is encountered... or possibly broken open by accident."

On Tuesday staff at the BBC were also sent an internal e-mail advising what them to do if they received a suspect package.

The "precautionary" missive warned staff to beware of packages, especially if they were unexpected and "appeared to contain powder or other unusual contents".